The ceramic industry depends on the use of binders to facilitate the fabrication of ceramic bodies. In classical ceramic triaxial bodies, the clay acts as the binder to help form the body. However, advanced ceramic bodies, (ZrO.sub.2, SiC, Si.sub.3 N.sub.4, etc.) do not contain clay, and therefore binder materials must be used to impart workability and green strength to ceramic compacts during processing prior to sintering. Those binders used to impart workability must have several key properties including: uniform mixing with the ceramic composition, providing sufficient green strength to allow processing, not decomposing during processing of the green body and complete burn-out during sintering thus leaving minimal residual ash in the sintered body. Of these characteristics the most important is complete burn-out with minimal residual ash. Several processes for forming ceramic bodies and several binder materials have been either proposed or are in use to provide the workability necessary during forming of ceramic bodies, among these are the following.
U.S. Pat. No. 4,608,361 discloses a process for producing a sheet for a catalyst carrier for vapor-phase reaction which comprises mixing highly heat resistant ceramic fibers, organic fibers, and an organic binder to form a slurry, forming the slurry into a paper sheet, impregnating the paper sheet with a silicon compound converting the silicon compound to silica gel and heating the impregnated sheet to burn up the organic fibers and the organic binder.
U.S. Pat. No. 4,551,496 discloses a thermoplastically moldable ceramic composition comprised of from about 40 to about 60 percent by volume of a sinterable silicon carbide powder and a binder comprised of an organic acid and a copolymer of ethylene and from greater than about 12 to about 33 weight percent vinyl acetate, said organic acid having a melting point ranging from about 44.degree. C. to about 88.degree. C. and ranging from greater than about 18 to about 45 weight percent of the binder. The ceramic composition is thermoplastically molded into a body which is baked to remove the binder and then sintered.
U.S. Pat. No. 4,530,808 discloses a sintered SiC body produced by forming a thermoplastically moldable ceramic composition comprised of sinterable silicon carbide powder and binder, thermoplastically molding the ceramic composition into a body, embedding the body in nominally spherical particles having a density greater than 80% of the particle's theoretical density and being selected from the group consisting of polycrystalline silicon carbide, free carbon-coated polycrystalline silicon carbide and mixtures thereof, baking the embedded body to remove the binder therefrom, recovering and sintering the baked body. The binder is comprised of stearic acid and a thermoplastic of ethylene and 25 wt % vinyl acetate.
U.S. Pat. No. 4,496,506 discloses a method for shaping a green body of a ceramic powder by use of a specific organic binder, which is a modified polyvinyl alcohol comprising monomeric units expressed by the formula: ##STR1## in which R is a monovalent hydrocarbon group having at least 4 carbon atoms, Z is a divalent linking unit of --O-- or --O--CO-- and .alpha. is zero or one, in a limited mole fraction. Due to the strong binding power and lubricity of the binder polymer, the wet blend of ceramic powder with an aqueous solution of the binder has good workability in shaping and wet shaped body has good shape retainability. The shaped body after drying also has excellent mechanical strength to facilitate handling.
U.S. Pat. No. 4,474,731 discloses a process for sintering and densification of ceramic materials containing hydrocarbon materials to produce ceramic bodies substantially free from carbonaceous residues of such hydrocarbon materials by compacting ceramic particles and a binder containing a pyrolysis catalyst selected from the group consisting of nickel and palladium ions, heating in a low oxygen atmosphere to a temperature in the range of 350.degree. to 780.degree. C. to remove the carbonaceous residue and further heating the material to produce a sintered, dense ceramic material.
U.S. Pat. No. 4,353,958 discloses a process for preparation of green ceramic tapes which comprise a finely divided ceramic powder and a water-compatible polyurethane resin binder. The green tapes are prepared by casting a water-based slip containing the ceramic powder and the water-compatible polyurethane resin binder in the form of a tape and then drying the formed tape. The drying is conducted in two stages, in the first a relatively humid atmosphere is maintained, in the second the humidity is reduced.
U.S. Pat. No. 4,346,049 discloses a process for the preparation of pressureless sintered silicon ceramic bodies having an equiaxed microstructure and an alpha crystalline habit. These ceramic bodies are produced by firing shaped bodies containing finely divided silicon carbide, a boron source such as boron carbide, a carbon source such as phenolic resin and a temporary binder such as polyvinyl alcohol at sintering temperatures from about 1900.degree. to about 2250.degree. C. under conditions such that a coating of carbon source is maintained on the finely divided silicon carbide.
U.S. Pat. No. 4,158,688 discloses a sacrificial binder for use in making molded ceramic bodies. The sacrificial binders used in this invention comprise block polymers having the structural formula: EQU X--[B(AB).sub..eta. A].sub..eta.'
wherein x is the linking group, A or B, .eta. is 0 or a positive integer, .eta.' is a positive integer greater than 2, A is a linear or branched polymer that is glassy or crystalline at room temperatures and has a softening point in the range from about 80.degree. to about 250.degree. C. and B is a polymer different from A that behaves as an elastomer at processing temperatures, a plasticizer which may be oil, wax, or oil and wax, and optionally other components such as supplementary resins, elastomers and antioxidants. Examples of suitable A materials include polystyrene, poly(acrylonitrile) poly(p-bromostyrene) poly(methyl methacrylate), poly(alphamethylsytrene) poly(2-methyl-5-vinylpyridine), poly(4-vinylpyridine), poly(vinyl acetate), polyesters, polyamides, polyurethanes, poly(vinyl chloride), polypropylene, polysulfones, poly(phenylene sulfide) poly(4-methyl pentene-1) and poly(vinyl alcohol). Examples of suitable B materials include polybutadiene, polyisoprene, polydimethylbutadiene, poly(ethylene oxide), poly(isopropyl acrylate), poly(octamethylcyclotetrasiloxane), poly(tetrahydrofuran), polyisobutylene, ethylene propylene rubber, ethylene propylene diene terpolymers, chlorosulfonated polyethylene, epichlorohyrin rubber, butyl rubber, chlorobutyl rubber, bromobutyl rubber, fluorocarbon rubbers and silicon elastomers.
As mentioned earlier, the most important requirement for a binder material is the ability to be completely burned-out while leaving minimal residual ash in the ceramic body. If a binder leaves residual ash this can significantly affect the mechanical, chemical or electrical properties of the ceramic body. Because of this, the burn-out operation is carried out in an oxidizing atmosphere. However, some of the newer ceramic compositions, e.g. Si.sub.3 N.sub.4 and SiC cannot be exposed to air. Therefore there is a growing need in the ceramic industry for a binder which will decompose in inert or reducing atmospheres. The above processes and binders do not present a practical answer to this need.